Use the nuclear option to solve the hung problem:

Besides deadlocks, a thread can in theory get stuck in a blocking pop(),
not seeing the stopping flag in particular. So assure liveness by making all
pop() timed pop.

src/process/FFTDataDistributor.cpp

@@ -5,6 +5,9 @@
 #include <algorithm>
 #include <ThreadBlockingQueue.h>
 
+//50 ms
+#define HEARTBEAT_CHECK_PERIOD_MICROS (50 * 1000) 
+
 FFTDataDistributor::FFTDataDistributor() : outputBuffers("FFTDataDistributorBuffers"), fftSize(DEFAULT_FFT_SIZE), linesPerSecond(DEFAULT_WATERFALL_LPS), lineRateAccum(0.0) {
 
 }
@@ -29,7 +32,10 @@ void FFTDataDistributor::process() {
 			return;
 		}
 		DemodulatorThreadIQDataPtr inp;
-		input->pop(inp);
+
+        if (!input->pop(inp, HEARTBEAT_CHECK_PERIOD_MICROS)) {
+            continue;
+        }
 
 		if (inp) {
             //Settings have changed, set new values and dump all previous samples stored in inputBuffer: 

src/process/SpectrumVisualProcessor.cpp

@@ -4,6 +4,8 @@
 #include "SpectrumVisualProcessor.h"
 #include "CubicSDR.h"
 
+//50 ms
+#define HEARTBEAT_CHECK_PERIOD_MICROS (50 * 1000) 
 
 SpectrumVisualProcessor::SpectrumVisualProcessor() : outputBuffers("SpectrumVisualProcessorBuffers") {
     lastInputBandwidth = 0;
@@ -194,7 +196,9 @@ void SpectrumVisualProcessor::process() {
 
     DemodulatorThreadIQDataPtr iqData;
     
-    input->pop(iqData);
+    if (!input->pop(iqData, HEARTBEAT_CHECK_PERIOD_MICROS)) {
+        return;
+    }
     
     if (!iqData) {
         return;

src/process/VisualProcessor.h

@@ -101,7 +101,7 @@ protected:
     //available outputs, previously set by attachOutput().
     //* \param[in] timeout The number of microseconds to wait to push an item in each one of the outputs, 0(default) means indefinite wait.
     //* \param[in] errorMessage an error message written on std::cout in case pf push timeout.
-    void distribute(OutputDataTypePtr item, std::uint64_t timeout = BLOCKING_INFINITE_TIMEOUT, const char* errorMessage = "") {
+    void distribute(OutputDataTypePtr item, std::uint64_t timeout = BLOCKING_INFINITE_TIMEOUT, const char* errorMessage = nullptr) {
       
         std::lock_guard < std::recursive_mutex > busy_lock(busy_update);
         //We will try to distribute 'output' among all 'outputs',